Over the weekend, an Optus spokesperson said that the telco would "start blocking the Interpol list later this month".

Optus has previously described the Interpol filter as "a safe, credible and tested approach, which has been implemented in other countries with proven results".

The Interpol list is believed to have been in use for a number of years, with telcos such as BT, O2 and Virgin having blocked addresses on it from reaching customers for some time. For a site to get onto the list, law enforcement agencies in at least two separate jurisdictions have to validate the entry as being illegal and not just potentially offensive. In addition, the age of children depicted through content on the sites must be younger than 13 years of age, or perceived to be less than 13.

"Optus can confirm that it will honour its commitment to block child sexual abuse material on the web," Optus general manager of regulatory compliance Gary Smith said in a statement last week, when the telco first confirmed its intention to implement the Interpol list.

"Optus will work with the AFP to implement the Interpol 'Worst of' list — an approach which blocks the worst of the worst child sexual abuse material. Optus will work with the IIA and other ISPs to develop a code based on the framework released today by the IIA."

It is not yet clear whether other ISPs will follow Telstra and Optus in implementing the Interpol filter. Primus is still evaluating it, while others, such as iiNet and Internode, have merely stated that they will comply with the law when it came to filtering.

"As always, Internode's position is that it will continue to do what it is lawfully obliged to do," Internode said last week.

"Throughout the filtering debate, iiNet has maintained it would always cooperate with law enforcement agencies," iiNet said.

The limited voluntary filtering initiative is a measure that ISPs and the Federal Government agreed to in mid-2010. The initiative was intended to operate while a review was carried out into the Refused Classification category of content, which the government's wider mandatory filter project is slated to block.